Basic processes for forming quinacridones are well known and described, for example, in U.S. Pat. Nos. 3,157,659; 3,256,285; and 3,317,539. (See also "Quinacridones", S. S. Labana et al., Chemical Review, 67, 1 through 18 (1967). U.S. Pat. No. 3,257,405 describes a method for preparing quinacridones involving the thermally induced ring closure of 2,5-dianilinoterephthalatic acid precursors or aniline-substituted derivatives thereof in the presence of polyphosphoric acid. The crude quinacridones that result from such processes must be subjected to additional finishing steps to modify the particle shape, crystal structure, and particularly the particle size, to render the quinacridones suitable for use as pigments.
Reducing the particle size of a quinacridone pigment in a coating composition, particularly a paint, leads to a more transparent (as opposed to opaque) finish. Particle size also affects the tinctorial strength and rheological properties of the pigment. Particle size can be controlled by steps taken during synthesis or by post-synthesis treatment ("conditioning"). Methods of controlling quinacridone particle size during synthesis have involved drowning of the quinacridone melt resulting from the ring closing reaction in water and/or alcohol, as described, for example, in U.S. Pat. No. 3,257,405 discussed supra. Another method for regulating quinacridone particle size, as discussed, for example, in U.S. Pat. No. 3,386,843, involves the addition of a particle size growth inhibitor which prevents the growth of crystals during isolation of the quinacridone. The addition of certain quinacridone derivatives to the ring closure step has also been reported. U.S. Pat. No. 5,368,641, for example, discloses the addition of various quinacridone derivatives as crystal growth inhibitors. (See also U.S. Pat. No. 5,457,302). U.S. Pat. No. 5,755,873 describes the introduction of a precursor of a sulfonyl-containing quinacridone additive (e.g., a sulfonyl-containing derivative of 2,5-dianilinoterephthalic acid, a sulfonyl-containing derivative of 2,5-dianilino-3,6-dioxo-1,4-cyclohexadiene-1,4-dicarboxylic acid and/or a sulfonyl-containing derivative of 2,5-dianilino-terephthalic acid) into the cyclizing step. Said precursors are described as being converted to the corresponding sulfonyl-containing quinacridone crystal growth inhibitor additive during the ring closure step.
Post-synthesis conditioning conventionally requires solvent treatment and/or ball milling of the crude quinacridone in the presence of a large amount of an inorganic salt, or premilling in a ball mill followed by homogenization with an organic paste. U.S. Pat. No. 5,084,100 describes the ball milling of a crude quinacridone in the presence of aluminum sulfate and esters of alkane dicarboxylic acid, which function as crystallizing solvents. Subsequent to ball milling, these solvents must be removed from the pigment. Another means for conditioning involves acid pasting in which the crude quinacridone is dissolved in a large quantity of concentrated sulfuric acid to form a solution, which is then drowned in water. Particle size growth inhibitors may also be introduced during conditioning. U.S. Pat. No. 4,455,173 describes the acid pasting or ball milling of a quinacridone in the presence of a 2-phthalimidomethyl quinacridone particle size growth inhibitor.
Notwithstanding the availability of the above-described methods, there has remained a need for quinacridone pigments providing improved transparency and rheological properties as well as excellent coloristic properties. Furthermore, a need remains for a process for preparing such quinacridone pigments that does not require the use of large amounts of hazardous concentrated acids, or environmentally unfriendly organic solvents.